Heretofore, there is known an image display apparatus having a light source and a rear projection screen that projects image light emitted from the light source in enlargement, as a typical image display apparatus represented by a rear projection type television. Further, there is generally known a rear projection screen composed of a Fresnel lens sheet combined with a lenticular lens sheet (diffusion sheet) as the rear projection screen used in the image display apparatus.
The Fresnel lens sheet adjusts image light emitted from a light source to make it to approximate parallel light. There is also a Fresnel lens sheet which collects or diverges light from a light source to a light outgoing side, in addition to the Fresnel lens sheet that adjusts light from the light source to make it to parallel light. In the description, however, explanation is made supposing that light emitted from a Fresnel lens sheet is approximately parallel light.
The lenticular lens sheet causes approximately parallel light adjusted by the Fresnel lens sheet to outgo to an observer as diverged light so that the observer can observe an image on a rear projection screen at various angular positions. More specifically, the lenticular lens sheet diffuses the approximately parallel light outgoing from the Fresnel lens sheet mainly in a horizontal direction as well as also in a vertical direction.
Incidentally, there is known a lenticular lens sheet having a plurality of unit lens portions each having a total reflection surface formed on a part of a side thereof, as the lenticular lens sheet described above. Note that light absorbing portions are formed between the unit lens portions as necessary to prevent the reflection of external light.
More specifically, there is known a lenticular lens sheet having a plurality of unit lens portions disposed thereto, as a first lenticular lens sheet, each of the unit lens portions having a curved surface formed on a light outgoing side vertex and total reflection surfaces formed to a part of side surfaces (refer to Japanese Patent Application Laid-Open Nos. 57-165830, 62-108232, and 60-159733).
Further, there is known a lenticular lens sheet having a plurality of unit lens portions disposed thereto, as a second lenticular lens sheet, each of the unit lens portions having a curved surface formed on a light outgoing side vertex as well as total reflection surfaces formed to a part of side surfaces, and at least two types of unit lens portions, each of which has a different tilt angle of the total reflection surface and a different lens height, being combined as one unit (refer to Japanese Patent Application Laid-Open Nos. 59-140434 and 59-68726).
However, since each unit lens portion of the first and second lenticular lens sheets described above has the curved surface formed on the light outgoing side vertex, a problem arises in that a support sheet cannot be joined to the unit lens portions to support them. Further, a problem also arises in that a lens is liable to be scratched and made dirty because a light outgoing surface of the lenticular lens sheet is formed in an irregular shape due to the curved surfaces of the unit lens portions, and thus the surface thereof cannot be wiped by hand.
Further, since a light absorbing portion cannot be formed to absorb and shield external light in the second lenticular lens sheet described above, the second lenticular lens sheet is disadvantageous in that when an observer views a display, it is difficult for an observer to observe an image because a contrast is low in a bright portion and a dark portion. Further, the second lenticular lens sheet is also disadvantageous in that it is difficult to enhance the resolution of an image by miniaturizing a lenticular lens because the second lenticular lens sheet is formed in a complex shape.
To cope with the above problems, heretofore proposed is a lenticular lens sheet having a plurality of unit lens portions disposed in parallel with each other, each of the unit lens portions having total reflection surfaces formed on the side surfaces of each approximately trapezoidal column, as a third lenticular lens sheet (refer to Japanese Patent Application Laid-Open Publication No. 2002-006112). However, the third lenticular lens sheet has the following problems, which will be explained below in detail.
As shown in FIG. 18A, light, which is emitted from a light source (not shown) and adjusted by a Fresnel lens sheet (not shown) so that it is made to approximately parallel light, is incident on a lenticular lens sheet 700. A part of the light, which is incident on the lenticular lens sheet 700 as described above, travels linearly through the lenticular lens sheet 700 and outgoes therefrom (L2′). However, the other light is reflected on one side of a trapezoidal columnar unit lens portion, refracted on a light outgoing side thereof, and outgoes therefrom (L1′). On the other hand, the other light is also reflected on the other side of the trapezoidal columnar unit lens portion, refracted on the light outgoing side thereof, and outgoes therefrom (L3′). As described above, the light outgoing from trapezoidal columnar unit lens portions disposed to the lenticular lens sheet 700 forms the three groups of light (L1′, L2′, L3′). Accordingly, when the brightness (gain) of an image formed by the light outgoing from the lenticular lens sheet 700 is measured with respect to a horizontal observation angle, there is obtained a gain curve which has a steep slope with three peaks corresponding to the groups of light (refer to FIG. 18B).
When an observer views an image displayed on a rear projection type television using the lenticular lens sheet 700 arranged as described above from the front of the television, the observer views the central portion of the television with the brightness at the position of 0° in the gain curve. In contrast, the observer views both the right and left ends of the television with the brightness at the positions of ±7.6° in the gain curve when both the ends are viewed at the position five times the height of a screen apart from the television, supposing that the television is a mass-produced television having a screen whose aspect ratio is set to 3:4, although the brightness is different depending on the distance of a position apart from the television where the image is viewed. Accordingly, in the lenticular lens sheet having a distribution of brightness (gain curve) including the steep slope with the three peaks, the observer views an image having bad bright uniformity. When, for example, the observation angles at both the right and left ends of the rear projection type television are approximately in agreement with an angle showing a minimum value of the gain curve, the difference between the brightness in the central portion of the television and the brightness at both the ends thereof extremely increases, thereby the observer views an image having extremely bad bright uniformity.
Further, when the observer views an image on the rear projection type television using the lenticular lens sheet 700 while moving in a horizontal direction from one end of the television to the other end thereof, the observer views the image, which extremely changes in the sequence of “dark, bright, dark, bright, dark, bright, dark” in the central portion of an image surface of the television. At the time, a problem arises in that an image having good bright uniformity cannot be viewed because the bad bright uniformity described above is also added.
Note that when the third lenticular lens sheet as described above is actually manufactured, a metal mold (mold) is used in consideration of mass production. Ordinarily, the metal mold is manufactured by sequentially forming trapezoidal columnar recesses (grooves) corresponding to unit lens portions on a mold material such as a metal substrate and the like by cutting it from an end thereof.
However, when the metal material is cut by the above method, a metal mold 705 is manufactured in such a shape that projections formed between adjacent recesses tilt from the sides of subsequently cut recesses toward the sides of previously cut recesses, as shown in FIG. 19A. When a lenticular lens sheet is manufactured using the metal mold 705 arranged as described above, a side segment, where the recess is formed previously, of the section of each trapezoidal columnar unit lens portion draws a curved line projecting inward, and the other side segment of the section draws a curved line projecting outward as shown in n FIG. 19B. Explained above is the example arranged such that the projections, which are made by forming the recesses for the unit lens portions by cutting the metal material, are deformed (plastically deformed) outward when viewed from the recesses being processed (FIG. 19A). However, there is also a case that a metal mold is manufactured so as to have such as a shape that projections formed to a metal material are deformed (elastically deformed) inward when viewed from recesses being processed, and projections formed between adjacent recesses tilt from the sides of recesses formed previously toward the sides of recesses formed subsequently, adversely to the above case. Note that a lenticular lens sheet 701 having the same shape as that shown in FIG. 19B can be manufactured even by the metal mold manufactured in the latter case.
Note that, when the lenticular lens sheet 701 manufactured as described above is used, a part of light incident thereon after it is adjusted and made to parallel light travels linearly and outgoes therefrom (L1″) as shown in FIG. 19B. Further, when the light, which is incident on the lenticular lens sheet 701 as described above, is reflected on the outwardly curved side surface of each trapezoidal columnar unit lens portion, it outgoes mainly at an angle near to a center (L2″), and when the light is reflected on the inwardly projecting side surface, it outgoes mainly at an angle apart from the center (L3″). As a result, a gain curve of the lenticular lens sheet 701, which shows the brightness (gain) of an image to a horizontal observation angle, is a right/left asymmetrical curve as shown in FIG. 19C. Note that the terms “right” and “left” used here correspond to “right” and “left” when the lenticular lens sheet 701 is assembled and used in the rear projection screen for the rear projection type television.
When the observer views an image between, for example, the light outgoing angles L1″ and L3″ in the rear projection type television using the lenticular lens sheet 701, he or she views the image in a dark state. Accordingly, when the observer views the image while moving in a horizontal direction from one end side to the other end side of the rear projection type television using the lenticular lens sheet 701 (while moving from the right side to the left side of FIG. 19B), the image is viewed in the sequence of “dark, bright, dark, bright, dark, bright” at the center of a screen, from which a problem arises in that it is difficult to view the image because irregular brightness is caused in the image. Further, when the observer views the image displayed on the rear projection type television using the lenticular lens sheet 701 from the front, a problem arises in that he or she views the image having a bad right/left balance, that is, bad bright uniformity.